1. Field of the Invention
The present invention relates to a process for preparing oxidized polyethylenes having acid numbers of from 10 to 40 mg of KOH/g by oxidation of a finely divided polyethylene of high density at below the softening point of the polyethylene and above 100.degree. C. with an oxidation gas of air and/or oxygen, the polyethylenes used containing from 0.015 to 0.5% by weight of a free radical forming organic compound in admixture.
Such processes give oxidized polyethylenes which are of interest for use as lubricants in the extrusion of polyvinyl chloride.
2. Description of the Related Art
It is known that polyethylenes can be oxidized by the action of free oxygen at elevated temperatures, oxidized products being obtained through chain degradation. This method is described in U.S. Pat. Nos. 3,293,112, 3,322,711 and 4,459,388 and in GB Patent 1,087,915. The oxidation is in general carried out at temperatures just below the melting point of the polyethylene, on the one hand to obtain as high a rate of reaction as possible, but on the other to prevent sticking or clumping of the pulverulent product.
In these processes, an accelerant is used, in small amounts of for example 0.05 to 5% by weight, based on the polyethylene to shorten the induction period and to raise the rate of oxidation. The accelerants proposed are either organic peroxides or azobisisobutyronitrile and the peroxides, conveniently dissolved in an organic solvent, are to be distributed over the polyethylene. The solvent is then evaporated off before the oxidation. This gives a uniform distribution of the peroxide in the polyethylene.
The prior art also discloses converting polyethylene of high density with air in the presence of ozone into oxidized polyethylenes (cf. GB Patents 951,308, 997,135 and 1,087,914).
The existing processes for oxidizing polyethylenes using peroxide accelerants have the disadvantage that the acceleration of the oxidation reaction is only moderate. A more effective accelerant than peroxides, in particular dibenzoyl peroxide, is ozone, admixed in the oxidation gas, which is usually air. However, the oxidized products obtained with ozone are not suitable for use as lubricants in the extrusion of polyvinyl chloride, since their thermostability is not sufficient. In addition, the material obtained sticks too early to the rolls.